This invention relates to electronic digital communication systems and more particularly to radiotelephone systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards. This application focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users in the downlink (DL) direction and the users implement the commands in the uplink (UL) direction and vice versa. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a WCDMA communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs calls to and from user equipments (UEs), such as mobile stations (MSs), via the appropriate base station(s) (BSs), which communicate with each UE through DL, or forward, and UL (i.e., mobile-to-base, or reverse) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS, which is called a Node B in 3GPP parlance, serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
A BS in a WCDMA system may use a primary scrambling code and one or more so-called secondary or alternative scrambling codes, each of which can be used with several channelization codes. Orthogonal variable spreading factor (OVSF) channelization codes are used in order to maintain link orthogonality while accommodating different user data rates. The OVSF scheme is a kind of code tree, in which each level in the tree is a set of codes that are mutually orthogonal and have the same SF. Since the chip rate in a direct-sequence CDMA system is typically constant, a higher SF, corresponding to a different level in the code tree, generally corresponds to a lower information bit-rate.
High-speed downlink packet access (HSDPA) is an evolution of WCDMA communication systems that provides higher bit rates, e.g., up to more than 10 megabits per second (Mb/s), by using higher order modulation, e.g., 16-ary quadrature amplitude modulation (16-QAM), multiple spreading codes, e.g., up to fifteen codes with SFs of 16, and DL-channel feedback information. The DL-channel feedback information is information sent by a UE to a BS through the UL channel regarding the DL channel's quality. The BS providing HSDPA service is usually called the “serving” BS or cell, and the HS-channels in the DL are transmitted only from the HSDPA serving cell. The serving BS uses the DL-channel feedback information to optimize the DL modulation and coding for throughput.
Another evolution of WCDMA is Enhanced Uplink (EUL), or High-Speed Uplink Packet Access (HSUPA), that enables high-rate packet data to be sent in the reverse direction. Efficiency of the UL transmission and maximization of the available network capacity are achieved by carefully scheduling the UL transmissions of the usually many UEs in a cell. The serving BS informs the individual UEs of when they are allowed to transmit, and at which power level, so that the total power in the cell and the noise remain within the acceptable limits. The transmission power levels for the UEs and the permissions to transmit are transmitted from the serving BS by absolute and relative grant messages carried by enhanced absolute and relative grant channels (E-AGCH and E-RGCH). These messages and channels are described, for example, in 3GPP Technical Specification (TS) 25.309 V6.5.0, FDD Enhanced Uplink Overall Description Stage 2 (Release 6), December 2005, Section 9, and 3GPP TS 25.321 V6.7.0, Medium Access Control (MAC) Protocol Specification (Release 6), December 2005, Section 11.8.
UL absolute grant information is packaged in serving grant (SG) messages carried by the E-AGCH. According to the WCDMA standards, an SG message includes six bits sent over one transmission time interval (TTI). It will be appreciated, however, that the methods and apparatus described in this application can be used with other message formats in other types of communication system.
In a WCDMA communication system, the E-AGCH is assigned one OVSF code having SF=256, whereby 10 QPSK symbols per slot are transmitted. All SG messages are transmitted on the same channel, and individual SG messages are tagged with the signature of the targeted UE. An SG message is coded into 30 QPSK symbols, which are transmitted over 3 time slots during a TTI having a duration of 2 milliseconds (ms). If the duration of the TTI is 10 ms, the 3-slot SG message is repeated five times during the TTI.
The E-AGCH signal is generated according to 3GPP TS 25.212 V6.7.0, Multiplexing and Channel Coding (FDD) (Release 6), December 2005, Section 4.10. FIG. 2 depicts the coding process. As indicated by step 202, a UE-specific cyclic redundancy check (CRC) signature of 16 bits and 8 tail bits are appended to the 6 AG message bits xag1, xag2, . . . , xag6. In step 204, the 30-bit sequence y1, y2, . . . , y30 is then coded using a rate-⅓, constraint-length-9 convolutional code. Rate matching is applied in step 206 to the resulting 90 bits z1, z2, . . . , z90 to produce a 60-bit transmit sequence r1, r2, . . . , r60, which is mapped to the physical channel, i.e., modulated onto a sequence xk of 30 QPSK symbols spanning the 3 time slots that are transmitted as the E-AGCH in a WCDMA system.
A UE can monitor the E-AGCH for grant messages simply by reversing the coding steps depicted in FIG. 2. The UE then typically includes, among other things, a Viterbi decoder that produces a local version of the 30-bit sequence y1, y2, . . . , y30 for each TTI and a device for checking CRC bits to determine if each sequence is properly decoded and intended for the particular UE. In case of a match, the UE applies a received message as an SG command.
The UE must monitor the E-AGCH constantly for grant messages while an enhanced dedicated channel (E-DCH) is configured. As a result, the monitoring equipment in the UE must be active during every TTI. Most of this activity conveys no useful information to the UE because the frequency of sending SG messages to any given UE is usually low, and thus the energy consumed and resources utilized during most of the decoding activity are effectively wasted.